1. Field of the Invention
The present invention relates to a ceramic composite.
2. Description of the Related Art
A C/C composite has outstanding specific strength and specific modulus, and moreover, excellent heat resistance and chemical stability in a high-temperature region exceeding 1,000° C., so that the C/C composite has been widely industrially used, for example, in semiconductor producing equipment and hot press apparatus including aerospace applications. In the meantime, in an industrial material of this kind, a radiation factor and chemical stability (reactivity) are influenced by characteristics of an exposed portion of a surface thereof. The C/C composite is a composite of carbon fibers and a carbon matrix such as glassy carbon, pyrolytic carbon or graphite, so that it becomes difficult to obtain a high purity C/C composite depending on selection of raw materials, so that diffusion of contaminants may occur from a surface thereof.
For the above-mentioned reason, it has been proposed in the C/C composite that a ceramic coating such as a SiC coating or a pyrolytic carbon coating is formed on the surface thereof. The forming of such a coating modifies properties of a surface of a member using the C/C composite to be able to improve exothermic and endothermic characteristics or corrosion resistance. Further, the high-purity gas impermeable ceramic coating formed on the surface can make it possible to reduce or prevent diffusion of impurities contained in the C/C composite.
The ceramic composite used under a high-temperature environment is likely to cause an interlayer peeling phenomenon at an interface with the substrate due to the difference in the coefficient of thermal expansion between the substrate and the ceramic coating formed on a surface thereof while being heated from ordinary temperature to high temperature. For this reason, there has been proposed a conversion method in which a substrate and a surface layer portion form a silicon carbide layer as a continuous tissue to provide a functionally graded material (for example, see JP-A-H5-132384).
Further, it has also been proposed that a paper body obtained by papermaking of carbon fibers is impregnated with pyrolytic carbon by Chemical Vapor Infiltration (CVI), thereby forming a pyrolytic carbon coating (for example, see JP-A-2002-68851).
However, in the above-mentioned SiC coating obtained by the conversion method or the coating obtained by CVI of pyrolytic carbon, the coating effectively acts on interlayer peeling only at the interface between the ceramic coating and the substrate. When the substrate is a C/C composite aligned in two directions parallel to the ceramic coating or a laminated body of a plurality of layers, or when fibers constituting the substrate do not cross each other in a thickness direction, like a nonwoven fabric, the C/C composite can not have sufficient strength against the interlayer peeling in the inside thereof. For this reason, the interlayer peeling phenomenon sometimes occurs in the inside of the C/C composite by internal stress generated by the difference in thermal expansion between the ceramic coating and the C/C composite. A main cause for this is that a carbon fiber layer in the vicinity of a surface layer of the C/C composite expands and contracts following the ceramic coating, but the adhering force of the ceramic coating does not act on a deep layer distant from the surface layer (the inside of the C/C composite layer).